The invention relates to a circuit arrangement comprising a high-voltage power transistor, drive means for supplying a pulse-shaped switching signal to the base electrode thereof and a load impedance connected to the collector electrode of the high-voltage transistor and to a supply voltage source, the collector current of the high-voltage power transistor driven to the saturation state, supplied by the supply voltage source, being interruptable under the influence of the pulse-shaped switching signal applied thereto and the drive means comprising an additional transistor the emitter electrode of which is connected to the base electrode of the high-voltage power transistor and the collector electrode to the collector electrode of the high-voltage power transistor.
Such a circuit arrangement, which is, for example, suitable for use in the line deflection circuit of television receivers, is known from an article published in the periodical "Electronics" of June 9th, 1977, pages 192 and 194. In this article it is proposed to construct the high-voltage power switch of the circuit as a so-called Darlington pair, the additional transistor amplifying the switching signal originating from a driver stage and controlling the base of the actual high-voltage power transistor. An advantage of a Darlington pair is that the additional transistor which need not switch large currents has a much higher current gain factor than the high-voltage power transistor, so that the driver stage need not supply a high power. According to said article the base current of the additional transistor is in the order of magnitude of 200 mA, so that a driver transformer is no longer required.
The use of a driver transformer may, however, be desirable, for example for isolating the ground of the circuit from the line voltage supply. This applies in particular to switched-mode power supplies which also comprise a high-voltage power switch and which may be implemented in a similar manner as a line deflection circuit and/or in combination therewith. The coupling, by means of a transformer between the driver stage and the final stage, dissipates less energy than a resistance coupling, as then a resistor must be connected to a d.c. voltage source while the driver stage can be fed, in the case of line voltage isolation, from a lower d.c. voltage source.
However, such a transformer coupling is not without problems. When the collector current of the high-voltage power transistor, which is driven to the saturation state, is to be switched off, both the base and the collector layers thereof contain a very high excess of charge carriers which must be removed. This implies that the base current which flows backwards during switch-off is very large, so that the drive energy is of the same order of magnitude in both time intervals, that is to say the interval in which said transistor conducts and the interval in which it is switched off. If a Darlington pair is used, the drive energy therefor is much lower in said first interval, due to the current gain of the additional transistor, than the control energy in the second interval. From this it will be apparent that the drive by means of a driver transformer is not possible without further measures.
A further difficulty is the result of the fact that the drive of the high-voltage power transistor must not satisfy the same conditions during both intervals. During the interval in which the transistor conducts, the forward flowing base current must have a given variation so that the dissipation during switch-off, which is related to the number of accumulated charge carriers during said interval, remains low. In contradistinction therewith the base current flowing backwards during switch off must have a variation which is described in greater detail in Dutch Patent Specification No. 138,210 (PHN 2526). Due to the presence of the additional transistor, the design of the drive circuit of the high-voltage power transistor, disclosed in this Patent Specification, cannot be used without further measures.